We present a dilemma: either we consider the aptness criterion of Independence to be unnecessary, or we accept the distinction

between default and deviant behavior as outlined in the causal modeling literature. The current work addresses positions that advocate

for non-psychological solutions to resolve structural isomorphism,

explicitly focusing on McDonald’s solution, which introduces a new aptness requirement known as Evident Mediation, framed in purely structural terms.

We show that in cases where this new requirement introduces an additional variable that leads to a positive reversal of the causal verdict,

breaking therefore, the isomorphism between models under examination, there is a possible intervention on the added variable leading to a metaphysically

impossible scenario. Hence, the modified model violates the criterion of Independence. Ultimately, in the context of the inquiry, this does not satisfy Woodward’s criterion of Independent Fixability, which is necessary for models to count strictly as causal. In conclusion, although both sides of the dilemma present advantages and disadvantages, we underline that the structural solution of adopting McDonald’s aptness criterion outweighs its benefits. Until a more effective constraint that enhances Evident Mediation emerges to address these shortcomings, introducing the default/deviant distinction remains the more favorable approach to avoid structural isomorphism.